Re-inventing the Dynamic Microphone: The KSM8 Dualdyne™
The new KSM8 Dualdyne does things once thought impossible in a dynamic microphone. Product manager John Born shares some secrets from the design team's seven-year odyssey.
https://www.youtube.com/watch?v=aQhd_TlI5g8At the January 2016 NAMM Show, Shure came out with its first all-new dynamic microphone in nearly 50 years. The KSM8 Dualdyne™ is the end result of an advanced development project that began in 2009 under the code name Kohlrabi.
I was involved from beginning to end. Professionally, it was a rollercoaster experience. We solved a wide array of problems, from acoustical engineering and manufacturing to industrial design and environmental testing, and earned multiple patents along the way. Now that it's over, I'd like to share a few stories about what makes the KSM8 Dualdyne so special.
Inspired By a Vegetable?
Let's start with the original project's code name, Kohlrabi (yes, the leafy relative to the cabbage). Its shape inspired one of the key insights for the Dualdyne capsule: using a series of tubes to reverse the airflow. So when our project team was chartered, it was naturally dubbed Project Kohlrabi.
We had to simultaneously embrace and reject everything we knew about dynamic microphones.
To re-imagine a mature technology like the dynamic microphone is a huge project. It required us to simultaneously embrace and reject everything we knew. That meant having a deep understanding of all previous designs, notably the Unidyne® III capsule and shock mount system used in the SM58®. At the same time, we knew we were going to break pretty much everything, which resulted in us quickly realizing we couldn't allow ourselves to be bound by pre-existing tools, parts, and processes. We really did start from scratch.
Stretching the Laws of Physics
The dynamic microphone is a relatively simple machine. The parts are big: a magnet, the coil, air cavities, and magnet housing. There are no active electronics involved, so there's not a "get out of jail free" card. It's been pretty well optimized over the years. In order to change that, it's really you against physics. Fortunately, we had some ideas.
We started with the basics. One of our engineers, Roger Grinnip, went back to the Shure archives and pulled out the lab notebooks of legendary Shure engineers Ben Bauer and Ernie Seeler. That led to the Unidyne and Unidyne III cartridges found in the famous Shure 55 and legendary SM58 and SM57.
Fortunately, Roger loves math...he does math for fun. For Project Kohlrabi, he used those old lab notebooks to develop his own equations—over 300 pages of work—that proved we could develop our concept: a premium dynamic microphone with the key qualities listed below.
- Negligible proximity effect for natural sound and a large, deep "sweet spot"
- Textbook cardioid pattern, turning off-axis stage bleed into a pleasant, natural sound across all frequencies
- Accurate, flat frequency response
- No discernible handling noise
- Classic Shure ruggedness and reliability
Since this would be a premium microphone, we also needed an industrial design consistent with outstanding performance.
The Dualdyne Concept
Historically, all major Shure advances in dynamic microphone design stemmed from the Unidyne capsules. The genius of Unidyne is its ability to produce a consistent cardioid polar pattern using only one mic element. In fact, every directional dynamic microphone in the world today is based on the original Shure Unidyne patent.
In fact, every directional dynamic microphone in the world today is based on the original Shure Unidyne patent from 1939.
The Dualdyne concept takes that a step further. As the name implies, this all-new capsule uses two diaphragms, but not in the way one would expect.
Basically, a very thin diaphragm doesn't affect high frequencies, but lower frequencies are progressively blocked. By inserting that diaphragm into the rear-entry airflow, we could tune and control proximity effect. And with that control came another "lever" to pull in the acoustic design.
The difficult part was that in the process of physically fitting the second diaphragm into the element, you virtually break everything that makes a dynamic microphone what it is. So we spent the next six years fixing all the things we just broke.
Clearing Hurdles
The quad airflow system proved to be the solution to our problem. Where the placement of the second diaphragm "broke" the dynamic microphone, the kohlrabi tubes reconnected the system. The next challenge was how to manufacture it. There was no way to mold it as a single part. It had to be plastic, and a glued joint wasn't strong enough. Neither ultrasonic nor exterior laser welding would work, and there were other parts that had to be attached to the cartridge assembly.
Developing an entirely new laser welding process was the only way to make that part correctly in terms of visuals and functionality. This process welded from the inside out, giving us the airtight seal we required, the small size, and a discreet visual footprint. Four tiny mirrors bounce a laser off the inside of each tube to weld the two parts together.
Another challenge was the diaphragm—the thinnest, lightest dynamic diaphragm we've ever designed. Using high-speed video, we saw that it moved a little too much when it got an impulse-type shock. So we created and patented a new part, the Diaphragm Stabilization System, which prevents the diaphragm from rocking in an undesirable motion, reducing unwanted mechanical artifacts.
That's just a few of many examples of how far we went to stay true to our vision for this microphone.
Quality Control
At Shure, every product manager knows that any new product must pass the drop test along with a whole battery of environmental tests (over 80 of them). For a totally new product like KSM8 Dualdyne, passing the drop test—a 6-foot free-fall onto a hardwood floor, both in a mic stand and loose—is a huge challenge.
It works like this: Manufacture 10 microphones and run them through a test. If even one microphone fails, it's back to the drawing board. It forces us to develop processes that meet exacting standards and are reliably repeatable. Getting the KSM8 to pass the drop test took over a year. So it's as rugged as an SM58. Arguably even tougher.
Another problem we solved was moisture. Instead of using open-cell foam as an internal pop filter, we found a hydrophobic fabric to line the grille. And because the rear entry for sound is a diaphragm, the entire microphone capsule is a sealed system. With its hydrophobic grille liner, the KSM8 Dualdyne is virtually waterproof. In fact, we ran it under a shower head for 30 minutes, and not only did the mic continue to work, but also the frequency response and cardioid pattern remained unchanged. This enabled us to keep the grille size reasonable despite the size of the kohlrabi tube network and extra diaphragm.
Of course, the full range of Shure testing is even more intense, with tests for humidity, sweat, and temperature extremes. The KSM8 Dualdyne is the only microphone to survive our HALT chamber, the Highly Accelerated Life Test. This exposes the mic to a force of 60 Gs for 10 minutes, then freezes to below -70 degrees Fahrenheit, followed by heating to +200 degrees, repeating the process for hours. The KSM8 Dualdyne survived.
It took well over a year to fine-tune the KSM8 Dualdyne to the point where it would pass every physical, electrical, and acoustic standard that Shure requires.
I'll never forget the feeling of joy I had when we got there. When the quality lab came to inform me that our mic had passed every test, the first words out of my mouth were, "Don't touch anything!"
Our seven-year adventure was over.
A Microphone That Only Shure Could Make
Now, I admit that I'm more than a bit of a microphone geek, but trust me when I say the KSM8 Dualdyne offers a new tier of dynamic microphone performance. When you hold one in your hand, it feels substantial and looks beautiful. When you unscrew the grille and look inside, it's almost a piece of art. But the magic happens when you plug it in.
The frequency response is nearly flat, almost like a condenser. The off-axis rejection is uniform across all frequencies, so the bleed from cymbals, crowds, etc., sounds natural. If the talent wanders off microphone, only the volume changes; the tone remains the same.
For the team, the KSM8 Dualdyne was a labor of love. Our engineering group at Shure is second to none, and this is really their product. I just get to go along for the ride with our amazingly talented and brilliant engineering group. Shure is a special company that realizes how important it is to allow the time and resources to innovate in what seemed like a mature dynamic microphone category. That's what makes Shure different. And audio engineers, artists, performers, and audience members all benefit.
For a look inside how we make this awesome microphone, check out this video.
https://www.youtube.com/watch?v=Ddqt67YJr7k
